In a photolithography process, exposure of a photoresist to activating radiation is an important step in attaining a high resolution photoresist image. However, reflection of activating radiation from the photoresist and the underlying substrate substantially limits the resolution of a lithography process. Two major problems of reflected radiation are: (1) thin film interference effects or standing waves, which are caused by variations in the total light intensity in the photoresist film as the photoresist thickness changes; and (2) reflective notching, which occurs when the photoresist is patterned over substrates containing topographical features.
As semiconductor manufactures have sought to fabricate devices having a higher degree of circuit integration to improve device performance, it has become necessary to use photolithographic techniques using shorter wavelengths (300 nm or less in wavelength) in the deep and extreme ultraviolet (UV) spectra to achieve fine features. The use of shortened wavelengths of light for imaging a photoresist coating has resulted in increased reflection from the upper resist surface as well as the surface of the underlying substrate.
To reduce the problem of reflected radiation, prior art processes typically use a radiation-absorbing layer interposed between the substrate surface and the photoresist coating layer. Such an antireflective layer is also referred to as a bottom antireflective coating, i.e., BARC. Although BARC materials may effectively reduce the back reflection of activating radiation, removal of BARC materials without adversely interfering with the overlying photoresist and/or the underlying substrate proves to be challenging.
Most prior art BARC materials are designed to be removed by a plasma etch process (see, for example, U.S. Pat. Nos. 5,939,236, 6,503,689, 6,610,457, and 6,261,743). Plasma etching refers to the removal of material, typically a masked pattern of semiconductor material, by exposing the material to a plasma of chlorine- or fluorine-based etchants, which dislodges portions of the material from the exposed surface. Plasma etch processes often cause thinning of a photoresist layer. Thus, if the etch rate of the BARC materials and the photoresist layer is not well matched, the pattern in the photoresist layer may be damaged or not be transferred properly to the substrate. Plasma etch processes may also cause damage to the substrate thereby affecting the performance of the final device. Furthermore, the additional step of removing BARC materials increases cost and operational complexity in photolithography.
Other prior art BARC materials include wet developable BARC materials. For example, U.S. Patent Application Publication Nos. 2003/0166828 and 2004/0210034 disclose BARC materials based on polyamic acids. This type of BARC materials has to be baked at a certain temperature range to partially covert the polyamic acids into polyimides to obtain the desired dissolution properties. The required bake temperature window can be narrow (<10° C.) and difficult to be effectively controlled. Moreover, exposed and unexposed areas of this type of BARC materials have the same dissolution rate in an alkaline developer, often causing an undercut of the photoresist lines. U.S. Patent Application Publication No. 2003/0129531 describes a positive photoimaginable BARC material based on a polymer comprising at least one unit with an acid labile group. This type of BARC materials requires the BARC material and the overlying photoresist have matching photospeeds. U.S. Patent Application Publication No. 2003/0215736 describes a negative photoimaginable BARC material based on a polymer which undergoes crosslinking upon light exposure to obtain the desired dissolution property. However, the acid residues generated in the crosslinking process may diffuse to the photoresist/BARC interface causing adverse effects, such as footing in a negative photoresist.
Thus, there remains a need for an antireflective coating composition that is developable in an aqueous alkaline developer, compatible with the overlying photoresist, and has desired optical properties so that it can also be used as a BARC in short wavelength photolithography.